(545g) The Role of Geometric and Electronic Effects in Creating Active Sites On the Surface of Lead-Doped Bismuth Molybdates

Multicomponent bismuth molybdenum oxide catalysts have been used industrially for over 60 years for the oxidation and ammoxidation of light olefins. Despite their long history, little is understood about the role of each element, or how favorable interactions between elements lead to a better performing catalyst. The most heavily studied reaction in the literature, and that which we continue to investigate, is the oxidation of propene to acrolein. This reaction proceeds via a Mars and van Krevelen mechanism, where the rate-determining step is the initial abstraction of a hydrogen radical from propene^[1].

Many of the industrially used catalysts are multi-phase and contain on the order of 10 different elements, making them difficult to model computationally. However, there are a number of multicomponent single-phase systems based on the scheelite structure, the parent structure of α-bismuth molybdate, that have been studied experimentally. For example, Sleight et al. have studied a range of Pb_1-3xϕ_xBi_2xMoO₄ for the oxidation of propene to acrolein^[2]. They found that while pure scheelite PbMoO₄ is not active, doping with a small amount of bismuth and introduction of charge compensating defects (x~0.06) result in a catalyst with a higher activity than that of pure Bi₂(MoO₄)₃.

Recent advances have identified the active site for alpha hydrogen abstraction of propene on Bi₂(MoO₄)₃ to consist of a molybdenyl oxygen perturbed by a neighboring bismuth atom at ~3Å distance^[3]. The reaction barrier was found to be the spin crossing from the singlet to the triplet state, in which a molybdenum is reduced by one electron and the associated oxygen is activated in order to later abstract a hydrogen radical from the propene. The nearby bismuth both destabilizes the singlet state via repulsion with the lone pair and stabilizes the triplet via interaction of the Bi 6p and the Mo=O π* orbital, allowing this reaction to occur with a reasonable barrier.

The present work extends previous investigations in this area by further examining the effect of the neighboring element and the molybdate ion environment on the reaction barrier. Q-Chem calculations were performed on a variety of relevant clusters that mimic the environment in various scheelite structures. We find that both the distance and directionality of the neighboring element with respect to the active molybdenyl oxygen play key roles in its ability to perform the initial hydrogen abstraction. This finding is used to explain reaction barriers calculated in VASP using DFT+U density functional theory on a range of Pb_1-3xϕ_xBi_2xMoO₄ structures. The experimental inactivity of PbMoO₄ to propene oxidation is reproduced in VASP, and is due to the suboptimal geometry of the pure scheelite structure. However, the introduction of defects and bismuth into the scheelite PbMoO₄ distorts the structure to an improved geometrical configuration at the surface, resulting in a catalytically active material.

References:

[1] Grasselli R.K.; Burrington J.D. Industrial & Engineering Chemistry Product Research and Development 1984, 23, 393-404

[2] Sleight A.W.; Linn W. J Annals of the New York Academy of Sciences 1976, 272, 22-44

[3] Getsoian A.; Shapovalov V.; Bell A.T. Journal of Physical Chemistry C 2013, 117, 7123-7137